Selecting the Right Two-Component Dispense Valve

Many
factors need to be considered before the correct two-component dispense valve
can be selected for a meter/mix application(s). These include the material
viscosities, mix ratio (by volume), filler types and content, how the fillers
affect the specific gravity or density, the type of materials (e.g., silicones,
urethanes, epoxies or methacrylates), and the effect the chemistry can have on
each component that the material comes in contact with.

Additional questions to ask include: What type of application is this, a shot
(fill) or a bead application? Will the dispense valve be mounted on a robot,
gantry or pedestal-mounted so the part can be presented to the valve, or will
it be manually held? What is the flow rate of the material to be dispensed? All
of these questions will help you understand the material and application
requirements, as well as what valve is best designed for the specific process
demands.

In the process of dispensing and mixing the two components, it is critical that
both components are introduced into the mixer at precisely the same time,
on-ratio and at the correct flow rate. If these conditions are not met, the
mixed material may not cure, leaving a “soft spot” in the encapsulant or a
place where the material does not adhere. This can result in failed or scraped
parts and loss of production. With today’s tight delivery schedules and thin
operating budgets, no quality or production manager can afford these types of
problems.<

Chemistry

It
is essential to know the attributes of the materials to be dispensed; much of
this information can be found on the Material Safety Data Sheet (MSDS) and the
Tech Data Sheet (TDS), which are supplied by the material manufacturers.

Today, epoxies, silicones, urethanes, methacrylics, and other adhesive and
sealants offer a wide range of physical properties. Chemistries are constantly
being modified to meet a particular manufacturing process, and it is important
to have a good understanding of the material you are going to dispense before
selecting the dispense valve.

With so many different material chemistries available, consideration needs be
given to the components of the valve that will come in contact with the
material. Knowing the chemical composition of the materials that are to be
dispensed allows for the correct selection of the hard and soft goods found in
the dispense valves.

Some materials are corrosive or have aggressive components, while others may be
abrasive and can cause premature wear if not given proper attention. Certain
materials require a specific stainless steel to ensure that the curing agent is
not negated and the material rendered ineffective. Other materials will cure or
crystallize when exposed to ambient moisture. Proper handling of these
materials is commonly understood, but it is important to remain mindful and
avoid their exposure to moisture.

Viscosity

The viscosities of the materials to be dispensed can vary widely, from thinner
than water (1 cps) to as thick a heavy tar (greater than 1,000,000 cps). If you
are dispensing two thin or two thick materials, the materials can often feature
similar dispensing properties and affect the back-pressure and dispensing
process similarly. However, if the two materials have widely different
viscosities, like olive oil (84 cps) and peanut butter (250,000 cps), a new set
of conditions needs to be addressed.

It is critical that both materials come together in the mixer at the same time,
on-ratio and at the correct flow rate, so thought needs to be given to valve
selection. The lower-viscosity material will flow more readily and with less
back-pressure than the thicker material. It is therefore important to design
the dispense valve so the thinner material does not come out of the valve
sooner than the thicker material. This condition is called “lead-lag” and can
cause problems with the mixed material’s cure properties.

To resolve this difference, forethought must be given to the sizing of the
valve exit ports so that the back-pressure and flow rates of the two materials
allow proper flow and a uniform, homogenous mix. Finally, if the problem cannot
be resolved, a dual-valve, static-mix manifold can be used. Most two-component
valves are designed so the resin and catalyst are allowed to flow
simultaneously when the valve opens. The dual-valve, static-mix manifold is
designed so the resin and catalyst valves operate independently. With this
design, the valves can be opened at separate intervals to offset the lead-lag
condition.

Viscosity can also influence how the material stops flowing when the dispense
valve is shut off. Ideally, the flow of the material should stop cleanly once
the valve is turned off to prevent material waste and mess (such as if the
mixed material strings or oozes on the outside of the parts or on the process
line). Thinner materials that seek their own level need to be dispensed from a
drip-less ball-and-seat-style dispense valve, which shifts the ball onto the
seat to impede the flow of the material.*

Thicker materials that cause greater back-pressure in the mixing chamber and
can be stringy (or otherwise difficult to cleanly shut off) require a
spool-style valve.** This type of valve draws a spool back to shut off, which
eliminates the pressure in the mixer that would bleed out if it was not
reduced.

Ratio

The
material’s resin/catalyst ratio can vary from 1:1 to beyond 100:1. Not all
valves are designed to dispense ratios greater than 5:1, so it is important to
know the volumetric ratio of the material to be dispensed and make sure the
valve is capable of operating at that ratio.

Wide-ratio materials can present a unique set of conditions. Not only does the
dispense valve have to be designed to dispense wide ratios, but the metering
equipment needs to be capable of maintaining the correct ratio at the necessary
flow rate(s) to the dispense valve. Because one part of the material will flow
at a higher rate than the other, the size of the orifices in the valve outlet
needs to compensate for the difference in flow rate and the back-pressure.

The wide ratios can also create a problem with the proper mixing of the two
materials. Some wide-ratio applications can have a low flow rate as little as a
trickle and a high flow rate that is a steady stream. In this case, how and
where the low-volume side is introduced into the stream of the high-volume side
can have a significant impact on the mix quality of the two components; the
dispense valve has to be selected with that in mind.

Flow Rates

The flow or fill rate of the process dictates the size of the dispense valve.
As a result, the valve must be capable of providing the mixed material at the
volume needed for the process. If the sizing on the valve is too small, the
back-pressure can have an adverse affect on the metering equipment and could
possibly influence the performance and ratio of the equipment. Low-, standard-
and high-volume dispense valves are available to match to specific process flow
rate requirements.

Static-Mixing Valves

In the past, choices were limited when dispensing a two-component adhesive or
sealant. One option was to use a mix manifold block along with an inline static
mixer(s) that had to be solvent-flushed or resin-purged to prevent curing. The
other option was a dynamic mixer, but, like the mix manifold block, dynamic
mixers had to be broken down and cleaned at the end of production. Since the
introduction of static disposable mixers, the design of two-
component dispense valves has evolved a great deal. Today, static disposable
mixers are used in most applications and can be easily disposed of when
production is finished.

Static mixers are designed to work when the two materials are introduced into
the mixer from the dispense valve. They are cut in half and folded as they pass
over each individual mixing element so that
the material is completely blended by the time it passes through the
mixer. Static mixers are available with different numbers of elements in order
to address the necessary degree of mixing. In addition, a variety of diameters
are available that can be matched to the material being dispensed and the
process parameters. Static mixers can also be adapted for Luer-Lok needles and
high-pressure “stream” insets.

For processes that require additional blending, static mixers have been
designed for use on power static-mix (PSM) valves. With a PSM valve, the mixing
elements in a rotary static mixer are spun within the mixer tube. The mixers
can be properly disposed of during production breaks, thus alleviating the need
to break-down and clean the dynamic mixing valves.

PSM mixers are available with either pneumatic or servo-drive power heads.
Pneumatic power heads are typically set at the preferred rotational speed
necessary to achieve the desired material blend; servo-driven power heads
operate at higher speeds and offer the most consistent mixing speed and
material blend. To determine the proper static mixer size, tests typically have
to be run with the two-component material at the flow rate and shot size
required by the application.

Mixer Shrouds

Once the static mixer has been determined, the matching static mixer shroud can
be selected. The shroud serves several functions, such as retaining the mixer
on the dispense valve, keeping the mixer in a repeatable location, and
protection in the event that the mixer is partially cured and the force of the
incoming material causes the side of the mixer to burst.

Simple Selection

The process of selecting the correct dispense valve for two-component
dispensing applications can seem overwhelming. Simplify the selection process
by having all of your material and process information available when you
consult a valve expert. This will leave you with fewer worries and, ultimately,
a dispense valve configured for your specific application.

For more information regarding two-component dispense valves, contact the
author at (734) 459-8600 or mruhlmann@sealantequipment.com, or visit
www.SealantEquipment.com/Valves.

Products

The Handbook of Sealant Technology provides an in-depth examination of sealants, reviewing their historical developments and fundamentals, adhesion theories and properties, and today’s wide range of applications.